The present invention relates to anti-lock control method and apparatus for preventing the locking of the automotive wheels of a running vehicle during the braking thereof, and more particularly, to an improvement of anti-lock control method and apparatus for preventing braking force applied to inner wheels of the turning vehicle from being insufficient.
In a conventional anti-lock control apparatus for an automotive vehicle, a controller including a microcomputer performs anti-lock control in such a manner that the mode of controlling the brake hydraulic pressure is determined on the basis of an electric signal indicative of the wheel speed detected by a wheel speed sensor. In accordance with the determined mode, a hold valve, which is a normally-open solenoid valve, and a decay valve, which is a normally-closed solenoid valve, are opened or closed to increase, hold or decrease the brake hydraulic pressure of the automotive wheels in order to maintain the steering property and moving stability of the vehicle and shorten the braking distance thereof.
The present inventor once filed a U.S. patent application (Ser. No. 07/403,375) as a related art of the present invention. An anti-lock control method described in the U.S. patent application will be hereinafter described for the purpose of clear understanding of the present invention.
In the anti-lock control method, a reference speed Vr for pressure decrease judgment is set on the basis of the speed (hereinafter referred to as the wheel speed Vw) of each wheel controlled by brake control units for the four wheels of a vehicle. Then an estimated vehicle speed is set, which follows the highest wheel speed (four-wheel select-high speed VwH) selected among the wheel speeds Vw but the decrease of which, following the four-wheel select-high speed VwH, is limited to a prescribed range of speed decrease rate. Finally, a first and a second threshold speed VT1 and VT2, which follow the estimated vehicle speed Vv with prescribed speed differences therefrom, are set (Vv&gt;VT1&gt;VT2). Statuses of pressure increase, pressure hold and pressure decrease, which are described hereinafter, are set on the basis of the comparison of the wheel speed Vw with the estimated vehicle speed Vv and with the reference speed Vr or the threshold speeds VT1 and VT2. A hold valve HV and a decay valve DV, which are solenoid valves, are opened or closed in prescribed control manners set for the statuses, to perform the pressure increase, the pressure hold or the pressure decrease.
FIG. 7 is a control time chart indicating the changes in the wheel speed Vw and the brake hydraulic pressure Pw and the opening and closing of the hold valve HV and the decay valve DV in the anti-lock control.
The reference speed Vr is set as described hereinafter.
When the brake of the vehicle is not in operation during the movement of the vehicle, the brake hydraulic pressure Pw is not increased, the hold valve HV is open and the decay valve DV is closed. When the brake is put in operation, the brake hydraulic pressure Pw begins to increase at a time point A shown in FIG. 7 and rise sharply (in an ordinary mode) and the vehicle speed Vw is thereby decreased. The reference speed Vr is set so that it follows the wheel speed Vw with a speed difference 66 V therefrom so as to be lower than the wheel speed, and that the reference speed begins to rectilinearly fall with a speed decrease gradient .theta. of -1G when the deceleration dVw/dt (negative acceleration) of the wheel speed has reached a prescribed threshold value of -1G, for example, at a time point B.
Statuses shown in FIG. 7 are described from now on.